There are several techniques for molding parts, particularly made of plastics, composites or metal.
Injection molding has been a known practice for a long time. Two half-molds are brought toward one another and come into contact along a parting line. Then the material to be molded is rapidly injected into the mold. Since the half-molds are adjacent, it is possible to provide inside the cavity zones around which the injected material can spread. Once the mold is full, the injection continues in order to ensure a correct filling of the mold and to take account of the shrinkage of the material during cooling. However, this additional filling requires the application of a considerable counter-pressure on the half-molds in order to prevent the two half-molds from separating under the effect of the injection pressure. The separation of the half-molds would result “at best” in the appearance of burrs along the parting line, which is totally intolerable.
To prevent this problem, it is known practice to apply a counter-pressure to one of the half-molds with the aid of a press. The thinner the part and the greater its projected area, the greater the locking force the press must apply. Moreover, it is often preferable for the dimensions of the press to allow the application of a counter-pressure greater than the value that is in theory strictly necessary.
In reality, the manufacturer of molded parts—the “transformer”—usually has a fleet of presses in a given range of locking force. Depending on the counter-pressure necessary to manufacture the part, he chooses one of the presses from his fleet and installs the mold therein. Hitherto, the presses used have usually been powerful enough.
In order to remedy this pressure problem, the injection-compression molding technique has been developed. This involves placing a certain quantity of material to be molded in a first half-mold while bringing a second half-mold toward the first. Under the effect of the pressure, the slug of material is spread out inside the mold, between the two half-molds, and it closely matches the cavity thus defined.
However, this technique makes it impossible to provide openings at the parting line of the two half-molds. To obtain a part comprising openings, it is necessary to provide steps for cutouts in the molded part in order to make the desired openings.
Therefore, the quality of finish is not always satisfactory both from the point of view of the surface appearance of the molded parts and from the point of view of the cutouts to be made.